Body and Armature A-Pose / Clothes T-Pose
The goal can be reached having all 3 objects (Armature, body and armor) in a T-pose as Rest Pose.

This can be a procedure:

Move in pose mode the armature using FK (while it is deforming the body only) until you get the desired T-Pose; then adjust the IK controls so that there is nearly no difference between IK and FK.

Select the body and Apply its Armature modifier.

Select the armature, turn on all its bone layers, select all bones and click "Apply pose as rest pose" from the menu "Pose - Apply".

At this point you can parent the armor to the Armature, add a new Armature modifier to the body and set  the Armature as object.

Don't forget to move the new Armature modifier on top of the modifiers stack, and check its "Mantain volume" option.

This procedure doesn't affect any premade weight assignements.